Run 2 Monte-Carlo Works hop April 20, 2001 Rick Field - Florida/CDF Page 1 The Underlying Event in The Underlying Event in Hard Scattering Processes Hard Scattering Processes The underlying event in a hard scattering process is a complicated and not very well understood object. It is an interesting region since it probes the interface between perturbative and non-perturbative physics. It is important to model this region well since it is an unavoidable background to all collider observables. I will report on two CDF analyses which quantitatively study the underlying event and compare with the QCD Monte- Carlo models. The Underlying Event: beam-beam remnants initial-state radiation multiple-parton interactions Proton A ntiProton PT(hard) O utgoing Parton O utgoing Parton U nderlying Event U nderlying Event Initial-State R adiation Final-State Radiation CDF QFL+Cones Valeria Tano Eve Kovacs Joey Huston Anwar Bhatti CDF WYSIWYG+Rick Field David Stuart Rich Haas Ph.D. Thesis Ph.D. Thesis
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The underlying event in a hard scattering process is a complicated and not very well understood object. It is an interesting region since it probes the interface between perturbative and non-perturbative physics.
It is important to model this region well since it is an unavoidable background to all collider observables.
I will report on two CDF analyses which quantitatively study the underlying event and compare with the QCD Monte-Carlo models.
The Underlying Event:beam-beam remnantsinitial-state radiation
multiple-parton interactions
Proton AntiProton
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying Event Underlying Event
Initial-State Radiation
Final-State Radiation
CDFQFL+ConesValeria TanoEve KovacsJoey Huston
Anwar Bhatti
CDFWYSIWYG+
Rick FieldDavid Stuart
Rich Haas
Ph.D. ThesisPh.D. Thesis
Run 2 Monte-Carlo Workshop April 20, 2001
Rick Field - Florida/CDF Page 2
Require PT > 0.5 GeV, || < 1
Make an 8% correction for the track finding efficiency
Errors (statistical plus systematic) of around 5%
Zero or one vertex
|zc-zv| < 2 cm, |CTC d0| < 1 cm
Require PT > 0.5 GeV, || < 1
Assume a uniform track finding efficiency of 92%
Errors include both statistical and correlated systematic uncertainties
Look at charged particle correlations in the azimuthal angle relative to the leading charged particle jet.
Define || < 60o as “Toward”, 60o < || < 120o as “Transverse”, and || > 120o as “Away”. All three regions have the same size in - space, x = 2x120o = 4/3.
Charged Jet #1Direction
“Toward”
“Transverse” “Transverse”
“Away”
-1 +1
2
0
Leading Jet
Toward Region
Transverse Region
Transverse Region
Away Region
Away Region
Run 2 Monte-Carlo Workshop April 20, 2001
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Charged Multiplicity Charged Multiplicity versus Pversus PTT(chgjet#1)(chgjet#1)
Data on the average number of “toward” (||<60o), “transverse” (60<||<120o), and “away” (||>120o) charged particles (PT > 0.5 GeV, || < 1, including jet#1) as a function of the transverse momentum of the leading charged particle jet. Each point corresponds to the <Nchg> in a 1 GeV bin. The solid (open) points are the Min-Bias (JET20) data. The errors on the (uncorrected) data include both statistical and correlated systematic uncertainties.
Charged Jet #1Direction
“Toward”
“Transverse” “Transverse”
“Away”
Underlying Event“plateau”
Nchg versus PT(charged jet#1)
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<N
ch
g>
in
1 G
eV
/c b
in
1.8 TeV ||<1.0 PT>0.5 GeV
"Toward"
"Away"
"Transverse"
CDF Preliminarydata uncorrected
Run 2 Monte-Carlo Workshop April 20, 2001
Rick Field - Florida/CDF Page 5
Shape of an AverageShape of an AverageEvent with Event with PPTT(chgjet#1) = 20 GeV/c(chgjet#1) = 20 GeV/c
<Nchg> = 8.2
<Nchg> = 1.2
<Nchg> = 4.5
<Nchg> = 1.2
PT(charged jet#1) = 20 GeV
Includes Jet#1
Underlying event“plateau”
Remember|| < 1 PT > 0.5 GeV
Shape in Nchg
Nchg versus PT(charged jet#1)
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<N
ch
g>
in
1 G
eV
/c b
in
1.8 TeV ||<1.0 PT>0.5 GeV
"Toward"
"Away"
"Transverse"
CDF Preliminarydata uncorrected
Run 2 Monte-Carlo Workshop April 20, 2001
Rick Field - Florida/CDF Page 6
““Height” of the UnderlyingHeight” of the UnderlyingEvent “Plateau”Event “Plateau”
<Nchg> = 8.2
<Nchg> = 1.2
<Nchg> = 4.5
<Nchg> = 1.2
PT(jet#1) = 20 GeV
Implies 1.09*3(2.4)/2 = 3.9 charged particles per unit
with PT > 0.5 GeV.
Implies 2.3*3.9 = 9 charged particles per unit
with PT > 0 GeV which isa factor of 2 larger
than “soft” collisions.
Charged Particle Pseudo-Rapidity Distribution
0
2
4
6
8
10
-10 -8 -6 -4 -2 0 2 4 6 8 10
Pseudo-Rapidity
Herwig "Soft" MB Isajet "Soft" MB MBR CDF DATA
dNchg/d
1.8 TeV Proton-Antiproton Collisions
Hard
Soft4 per unit
Run 2 Monte-Carlo Workshop April 20, 2001
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““Transverse” Nchg Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)
Plot shows the “Transverse” <Nchg> versus PT(chgjet#1) compared to the the QCD hard scattering predictions of Herwig 5.9, Isajet 7.32, and Pythia 6.115 (default parameters with PT(hard)>3 GeV/c).
Only charged particles with || < 1 and PT > 0.5 GeV are included and the QCD Monte-Carlo predictions have been corrected for efficiency.
Pythia 6.115
Herwig 5.9
Isajet 7.32Charged Jet #1
Direction
“Toward”
“Transverse” “Transverse”
“Away”
"Transverse" Nchg versus PT(charged jet#1)
0
1
2
3
4
5
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
"Tra
ns
ve
rse
" <
Nc
hg
> in
1 G
eV
/c b
in
Herwig Isajet Pythia 6.115 CDF Min-Bias CDF JET20
1.8 TeV ||<1.0 PT>0.5 GeV
CDF Preliminarydata uncorrectedtheory corrected
Run 2 Monte-Carlo Workshop April 20, 2001
Rick Field - Florida/CDF Page 8
““Transverse” PTsum Transverse” PTsum versus Pversus PTT(chgjet#1)(chgjet#1)
Isajet 7.32
Pythia 6.115
Herwig 5.9
Charged Jet #1Direction
“Toward”
“Transverse” “Transverse”
“Away”
"Transverse" PTsum versus PT(charged jet#1)
0
1
2
3
4
5
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<P
tsu
m>
(G
eV
/c)
in 1
Ge
V/c
bin
Herwig Isajet Pythia 6.115 CDF Min-Bias CDF JET20
1.8 TeV ||<1.0 PT>0.5 GeV
CDF Preliminarydata uncorrectedtheory corrected
Plot shows the “Transverse” <PTsum> versus PT(chgjet#1) compared to the the QCD hard scattering predictions of Herwig 5.9, Isajet 7.32, and Pythia 6.115 (default parameters with PT(hard)>3 GeV/c).
Only charged particles with || < 1 and PT > 0.5 GeV are included and the QCD Monte-Carlo predictions have been corrected for efficiency.
Run 2 Monte-Carlo Workshop April 20, 2001
Rick Field - Florida/CDF Page 9
The Underlying Event:The Underlying Event:DiJet vs Z-JetDiJet vs Z-Jet
Charged Jet #1Direction
“Transverse” “Transverse”
“Toward”
“Away”
“Toward-Side” Jet
“Away-Side” Jet
Z-BosonDirection
“Transverse” “Transverse”
“Toward”
“Away”
“Away-Side” Jet
Charged Jet #1or
Z-Boson Direction
“Toward”
“Transverse” “Transverse”
“Away”
PT > 0.5 GeV || < 1
Look at charged particle correlations in the azimuthal angle relative to the leading charged particle jet or the Z-boson.
Define || < 60o as “Toward”, 60o < || < 120o as “Transverse”, and || > 120o as “Away”. All three regions have the same size in - space, x = 2x120o= 4/3.
Run 2 Monte-Carlo Workshop April 20, 2001
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Z-boson: Charged Multiplicity Z-boson: Charged Multiplicity versus Pversus PTT(Z)(Z)
Z-boson data on the average number of “toward” (||<60o), “transverse” (60<||<120o), and “away” (||>120o) charged particles (PT > 0.5 GeV, || < 1, excluding decay products of the Z-boson) as a function of the transverse momentum of the Z-boson. The errors on the (uncorrected) data include both statistical and correlated systematic uncertainties.
Nchg versus PT(Z-boson)
0
2
4
6
8
10
0 10 20 30 40 50 60 70 80 90 100
PT(Z-boson) (GeV)
CDF Preliminarydata uncorrected
1.8 TeV |eta|<1.0 PT>0.5 GeV
<Nchg>
"Toward" "Transverse"
"Away"
Z-bosonDirection
“Toward”
“Transverse” “Transverse”
“Away”
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DiJet vs Z-JetDiJet vs Z-Jet“Transverse” Nchg“Transverse” Nchg
Comparison of the dijet and the Z-boson data on the average number of charged particles (PT > 0.5 GeV, || <1) for the “transverse” region.
The plot shows the QCD Monte-Carlo predictions of PYTHIA 6.115 (default parameters with PT(hard)>3 GeV/c) for dijet (dashed) and “Z-jet” (solid) production.
DiJet
Z-boson
PYTHIA"Transverse" Nchg vs PT(charged jet#1 or Z-boson)
Tano-Kovacs-Huston-Bhatti HERWIG+QFL slightly lower at 1,800 GeV/c
agrees at 630 GeV/c.
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ISAJET: ISAJET: “Transverse” Nchg “Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)
Plot shows the “transverse” <Nchg> vs PT(chgjet#1) compared to the QCD hard scattering predictions of ISAJET 7.32 (default parameters with PT(hard)>3 GeV/c) .
The predictions of ISAJET are divided into three categories: charged particles that arise from the break-up of the beam and target (beam-beam remnants), charged particles that arise from initial-state radiation, and charged particles that result from the outgoing jets plus final-state radiation.
Beam-BeamRemnants
Initial-StateRadiation
ISAJETCharged Jet #1
Direction
“Toward”
“Transverse” “Transverse”
“Away”
Outgoing Jets
"Transverse" Nchg versus PT(charged jet#1)
0
1
2
3
4
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
"Tra
ns
ve
rse
" <
Nc
hg
> i
n 1
Ge
V/c
bin
1.8 TeV ||<1.0 PT>0.5 GeV
CDF Preliminarydata uncorrectedtheory corrected
Beam-Beam Remnants
Outgoing Jets +Final-State Radiation
Isajet Total
Initial-State Radiation
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Rick Field - Florida/CDF Page 17
PYTHIA: “Transverse” Nchg PYTHIA: “Transverse” Nchg versus Pversus PTT(chgjet#1)(chgjet#1)
Plot shows the “transverse” <Nchg> vs PT(chgjet#1) compared to the QCD hard scattering predictions of PYTHIA 6.115 (default parameters with PT(hard)>3 GeV/c).
The predictions of PYTHIA are divided into two categories: charged particles that arise from the break-up of the beam and target (beam-beam remnants); and charged particles that arise from the outgoing jet plus initial and final-state radiation (hard scattering component).
Beam-BeamRemnants
Outgoing Jetsplus
Initial & Final-StateRadiation
PYTHIACharged Jet #1
Direction
“Toward”
“Transverse” “Transverse”
“Away”
"Transverse" Nchg versus PT(charged jet#1)
0
1
2
3
4
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
"Tra
ns
ve
rse
" <
Nc
hg
> i
n 1
Ge
V/c
bin
1.8 TeV ||<1.0 PT>0.5 GeV
CDF Preliminarydata uncorrectedtheory corrected
Beam-Beam Remnants
Pythia 6.115 Total
Outgoing Jets +Initial & Final-State Radiation
Run 2 Monte-Carlo Workshop April 20, 2001
Rick Field - Florida/CDF Page 18
Hard Scattering Component:Hard Scattering Component: “Transverse” Nchg vs P “Transverse” Nchg vs PTT(chgjet#1)(chgjet#1)
QCD hard scattering predictions of HERWIG 5.9, ISAJET 7.32, and PYTHIA 6.115.
Plot shows the dijet “transverse” <Nchg> vs PT(chgjet#1) arising from the outgoing jets plus initial and finial-state radiation (hard scattering component).
HERWIG and PYTHIA modify the leading-log picture to include “color coherence effects” which leads to “angle ordering” within the parton shower. Angle ordering produces less high PT radiation within a parton shower.
Pythia uses multiple partoninteractions to enhacethe underlying event.
Proton AntiProton
Multiple Parton Interactions
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying EventUnderlying Event
Parameter Value
Description
MSTP(81) 0 Multiple-Parton Scattering off
1 Multiple-Parton Scattering on
MSTP(82) 1 Multiple interactions assuming the same probability, with an abrupt cut-off PTmin=PARP(81)
3 Multiple interactions assuming a varying impact parameter and a hadronic matter overlap consistent with a single Gaussian matter distribution, with a smooth turn-off PT0=PARP(82)
4 Multiple interactions assuming a varying impact parameter and a hadronic matter overlap consistent with a double Gaussian matter distribution (governed by PARP(83) and PARP(84)), with a smooth turn-off PT0=PARP(82)
Hard Core
Multiple parton interaction more likely in a hard
(central) collision!
and new HERWIG
!
Run 2 Monte-Carlo Workshop April 20, 2001
Rick Field - Florida/CDF Page 20
PYTHIAPYTHIAMultiple Parton InteractionsMultiple Parton Interactions
Plot shows “Transverse” <Nchg> versus PT(chgjet#1) compared to the QCD hard scattering predictions of PYTHIA with PT(hard) > 3 GeV.
The Underlying Event:The Underlying Event:Summary & ConclusionsSummary & Conclusions
The “Underlying Event”
The underlying event is very similar in dijet and the Z-boson production as predicted by the QCD Monte-Carlo models.
The number of charged particles per unit rapidity (height of the “plateau”) is at least twice that observed in “soft” collisions at the same corresponding energy.
ISAJET (with independent fragmentation) produces too many (soft) particles in the underlying event with the wrong dependence on PT(jet#1) or PT(Z). HERWIG and PYTHIA modify the leading-log picture to include “color coherence effects” which leads to “angle ordering” within the parton shower and do a better job describing the underlying event. HERWIG 5.9 does not have enough activity in the underlying event.
PYTHIA (with multiple parton interactions) does the best job in describing the underlying event.
Combining the two CDF analyses gives a quantitative study of the underlying event from very soft collisions to very hard collisions.
Proton AntiProton
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying Event Underlying Event
Initial-State Radiation
Final-State Radiation
Run 2 Monte-Carlo Workshop April 20, 2001
Rick Field - Florida/CDF Page 27
Multiple Parton Interactions:Multiple Parton Interactions:Summary & ConclusionsSummary & Conclusions
Multiple Parton Interactions
The increased activity in the underlying event in a hard scattering over a soft collision cannot be explained by initial-state radiation.
Multiple parton interactions gives a natural way of explaining the increased activity in the underlying event in a hard scattering. A hard scattering is more likely to occur when the hard cores overlap and this is also when the probability of a multiple parton interaction is greatest. For a soft grazing collision the probability of a multiple parton interaction is small.
PYTHIA (with varying impact parameter) describes the data very nicely! I need to check out the new version of HERWIG.
Multiple parton interactions are very sensitive to the parton structure functions. You must first decide on a particular PDF and then tune the multiple parton interactions to fit the data.